Human tears are secreted by the lacrimal gland and flow across the surface of the eye to a shallow pool, known as the lacrimal lake, located where the eyelids come together at their inner ends. From there, the tears drain through small openings in each of the upper and lower eyelids, termed the superior lacrimal punctum and the inferior lacrimal punctum, respectively. From the superior and inferior puncta, the tears pass into each of the superior and inferior lacrimal canaliculus, respectively, which are duct-like pathways that lead to the lacrimal sac. The lacrimal sac is the superior, expanded portion of the nasolacrimal duct, which drains tears into the nasal system. Active agents can thus be delivered to the nose and throat through the lacrimal canaliculi, which lead into the nasolacrimal duct.
Active agents frequently are administered to the eye for the treatment of ocular diseases and disorders. Conventional means for delivering active agents to the eye involve topical application to the surface of the eye. The eye is uniquely suited to topical administration because, when properly constituted, topically applied active agents can penetrate through the cornea and rise to therapeutic concentration levels inside the eye. Active agents for ocular diseases and disorders may be administered orally or by injection, but such administration routes are disadvantageous in that, in oral administration, the active agent may reach the eye in too low a concentration to have the desired pharmacological effect and their use is complicated by significant, systemic side effects, while injections pose the risk of infection.
The majority of ocular active agents are currently delivered topically using eye drops which, though effective for some applications, are inefficient. When a drop of liquid is added to the eye, it overfills the conjunctival sac, the pocket between the eye and the lids, causing a substantial portion of the drop to be lost due to overflow of the lid margin onto the cheek. In addition, a substantial portion of the drop that remains on the ocular surface is drained into the lacrimal puncta, diluting the concentration of the drug.